Spontaneous monolayer dispersion

Spontaneous Monolayer Dispersion of Oxides and Salts onto Surfaces of Supports Applications to Heterogeneous Catalysis... 

This article provides a review of various aspects of the phenomenon of spontaneous monolayer dispersion, namely, its nature, effects, and applications. It is based in the main on the work that has been carried out in our laboratory in the last 16 years. Relevant data and results from other laboratories have been included for discussion. 

To recapitulate we point out that Mo03 disperses spontaneously onto the surface of y-Al203 and forms a close-packed monolayer. 

We have extended our investigation to a great many systems of oxides and salts on supports with highly specific surfaces (II-14, 18-21). They all display the phenomenon of spontaneous monolayer dispersion. In Table I these systems are given along with the temperature and the period of time for a suitable heat treatment. 

Plots for these systems, similar to one shown in Fig. 3, have been obtained on the basis of the data from XRD quantitative phase analyses, but they are omitted here. Each plot contains a threshold dispersion capacity. The dispersion capacities so derived in our work are listed in Table II (Section II,G). We can see that within the limits of experimental error the dispersion capacities are either equal to or lower than the respective close-packed monolayer capacities. So we come to the conclusion that these compounds disperse spontaneously onto the surface of the support to form submonolayers more often than monolayers. 

Monolayer dispersion is a spontaneous process. Thermodynamics would require that a spontaneous process should proceed with diminishing free enthalpy G or AG < 0. Normally, a process that disperses a substance in a crystalline state as a monolayer or submonolayer, if not as a multilayer, onto the surface of a support would gain in entropy. If this process is energetically not so unfavorable as to reverse its trend, the free enthalpy would decrease and so occurs the spontaneity. Otherwise, the process of a crystalline substance dispersing as monolayer onto the surface of a support would not proceed at all. 

Similar results were reported for the same sample heated in a dry oxygen stream, and also for the system Mo03/Ti02 heated under 450°C either in a moist or dry oxygen stream. All these ISS studies are consistent with the spontaneous monolayer dispersion model. 

The active components of many commercial supported heterogeneous catalysts are oxides or salts. Even for many metal catalysts, the precursors of metallic particles are also oxides or salts in some dispersed form. Hence the preparation of heterogeneous catalysts is deeply concerned in one way or another about the dispersion of oxides or salts on support surfaces. Furthermore, promoters or additives added to heterogeneous catalyst systems are also oxides or salts. Therefore, the spontaneous monolayer dispersion of oxides or salts on supports with highly specific surfaces as a widespread phenomenon will find extensive application in heterogeneous catalysis. Examples illustrative of this viewpoint are cited in the following sections. 

It is worth mentioning that spontaneous monolayer dispersion is also a very useful scientific basis underlying the process of regeneration of deactivated metal catalysts. Supported metal catalysts may sinter during use at elevated temperatures. Sintering will cause the metal catalyst to lose initial activity, and in order to recover it one has to find an effective way to redisperse the metal on the catalyst support. Applying what we have learned from our studies on spontaneous monolayer dispersion to... 

Redispersion through an oxidation-reduction cycle as described previously is, indeed, an effective way to regenerate supported metal catalysts that have been deactivated because of sintering, and the underlying principle is spontaneous monolayer dispersion. 

Spontaneous monolayer dispersion of compounds on supports is a widespread phenomenon and displays many unique effects. The principles involved have applications not only to heterogeneous catalysis, but also to materials science and other related fields. The theoretical and practical aspects of this phenomenon appear to offer prospects that should not be overlooked. Studies in connection with this phenomenon are continuing in our laboratory. 

The authors acknowledge China s National Natural Science Foundation for generous support of this work, which is a part of the major project Structural Chemistry and Molecular Design. We are grateful to all our colleagues who have contributed toward a better understanding of the phenomenon of spontaneous monolayer dispersion. Thankful acknowledg-... 

Another important consideration in preparing mixed-oxide catalysts is the spontaneous monolayer dispersion of oxides and salts onto surfaces of support substrates on calcination. Both temperature and duration of calcination are important here, as discussed in the reviews by Xie and Tang [63] and by Knozinger and Taglauer [64]. If this dispersion step is inadequate or incomplete, the resulting oxide layer, and any reduced metal surface from it, will not be reproducible from the same catalyst system therefore, one can then have different catalysts prepared at different times and, of course, from one laboratory to another. Spreading and wetting phenomena in preparation of supported catalysts is discussed in Section A.2.2.1.3. 

Xie, Y. Tang, Y. Spontaneous monolayer dispersion of oxides and salts onto surfaces of supports applications to heterogeneous catalysis. Adv. Catal. 1990, 37, 1 3. 

When ionic or molecular crystalline solids are mixed with a support of high specific surface area and then heated at a suitable temperature below their melting point for several hours, many of them disperse spontaneously onto the surface of the carrier [ 173]. This can be detected by the disappearance of their X-ray diffraction peaks. The formation of a (sub-)monolayer of the guest compound on the surface of the support has been proposed, and it has been claimed that this process is driven by a gain in entropy [173-177]. As described below, this phenomenon can be utilized for the incorporation of oxides into the pores of zeo-htes. 

When these lipids are dispersed in water, they spontaneously form bilayer membranes (also called lamellae) which are composed of two monolayer sheets of lipid molecules with their hydrophobic surfaces facing one another and their hydrophilic surfaces contacting the aqueous medium. In the case of phospholipids such as phosphatidylcholine (10.50), the structure consists of ... 

So far we have elaborated on the phenomenon of spontaneous dispersion of oxide or salt as monolayer or submonolayer onto the surface of supports with highly specific surfaces. At this point one may well inquire into the nature of this interesting phenomenon. It is only natural for us to think about, first of all, the questions concerning the origin of spontaneity, the dispersion of oxide or salt as monolayer or submonolayer instead of multilayer, and the ubiquity of the phenomenon. 

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